#include<iostream>
using namespace std;
int main()
{
  double x;
  cin>>x;
  cout.precision(3);
  cout<<fixed;
  if(x>=0&&x<5)cout<< -x+2.5;
  if(x>=5&&x<10)cout<<2-1.5*(x-3)*(x-3);
  if(x>=10&&x<20)cout<<x/2-1.5;
}